The performance characteristics of a gas turbine engine depend in significant part on the maximum sustainable temperature with which it can be operated while avoiding damage to core components. The maximum sustainable temperature varies with the design of the engine but is fundamentally limited in any conventional design which uses core components constructed from metal alloys. Although such materials have excellent overall properties, their susceptibility to thermally-induced phenomena such as creep imposes an upper limit on operating temperature. This upper limit can be significantly increased by providing means for cooling temperature-limited portions (chiefly, turbine blades) of the engine core. However, even with the use of such expedients, maximum sustainable operating temperatures are typically several hundred degrees below that associated with a stoichiometric combustion process. Thus, the engine is typically powered by combustion of an air/fuel mixture which is more or less fuel-lean, depending on power requirements.
The above-described limitations have led to general recognition that the temperature problem must be solved if large improvements are to be made in the performance of gas turbine engines. This in turn has led to previous and continuing attempts to develop turbine engines that incorporate ceramic core components. Continued development is impeded by the fact that ceramics, although superior to metal alloys in terms of temperature resistance, are far more brittle. A ceramic component can be suddenly destroyed by various particles which may be propelled into the component by combustion gas. These particles may be small metal or ceramic chips dislodged from other components, or carbonaceous particles dislodged from accumulations thereof formed on an inner wall of the combustor. Fuel-rich zones within a combustor may result from a number of causes or may be inherent in the design thereof. Where these zones are sufficiently close to the inner wall of the combustor, carbon liberated by the combustion process accumulates thereon. During operation of the engine, particles dislodge from these accumulated deposits and are propelled by combustion gas into the turbine. The impact of these and the other forementioned particles with components such as turbine blades may result in breakage, and is a major source of failure in engines which are constructed with ceramic components.
Attempts at minimizing this source of failure have included the use of a scrolled duct between the combustor and turbine, whereby a vent opening formed in the radially outer wall of the duct provides for the possible escape of the particles to ambient air. This approach is viewed as unsatisfactory since some of the combustion gas escapes as well, thus lowering the efficiency of the engine.
A brute-force approach in axial-flow turbines has been to provide turbine blades that are sufficiently large and thick to withstand the impact of the particles without sustaining sudden breakage. This approach has the obvious drawbacks regarding weight and aerodynamic efficiency, and is of little value in small gas turbine engines such as those designed for automotive applications, wherein radial-inflow turbines are much more efficient.
Other approaches have focused on the geometry of the turbine wheel (rotor) blades in relation to the direction of inlet air, and have included angling the leading edge of the blades, thus effectively lowering the applicable vector of the impulsive force between the particle and the high-speed blade.
An objective of the present invention is to provide a gas turbine engine in which all or a significantly high percentage of the above-described particles are prevented from reaching the turbine.
Another objective is to provide a gas turbine engine in accordance with the forementioned objective while maintaining a substantially airtight plenum between the combustor and the turbine.
A still further objective of the invention is to provide a gas turbine engine that incorporates a particle trap generally positioned in a plenum extending from within the combustor to the turbine, the engine thereby being adapted to entrap and incinerate particles that dislodge from the inner walls of the combustor before the entrapped particles are propelled into the turbine.
These and further objectives and advantages of the invention will be apparent from the following description, which includes the appended claims and accompanying drawings.